Whay can’t ‘Russianness’ change through centuries also? I think you have never come across the idea of “Russia as the 3rd Rome” or the Byzantine Christian Empire which were in effect in the Tsarist Russia.
Russian ethnicity is of course still there, but nations are more a matter of ideas. Yes, the God-bearer Nation, the Third Rome, — that nation is now gone. A small remnant remains. We now have the New Historical Community of Men that Brezhnev proclaimed. When you look at the behavior in South East Ukraine: all this flocking to the Lenin monuments, the red flags, the militarism, the cult of the 1945 victory, — this is the Soviet Union spiritually. Has nothing to do with Russia.